While Sans makes more puns that will annoy his brother, He cares deeply about him, since they have a brotherly relationship. When Papyrus dies, Sans starts to stalk Frisk in grief, watching them what do they do.

Sans has some connection with WD Gaster he is the only one who remembers him and his brother Papyrus no, he was the real Asgore scientist .... Sans played WD Gaster in his own creation to protect his brother and that's how they grew up on Stardin.